Sintering Activation Energy Research Articles

Effect of CeO2 on sintering behavior, crystallization, and properties of CaO-Al2O3-SiO2 glass–ceramics for packages

The effect of CeO2 on the sintering behavior, crystallization, and properties of CaO-Al2O3-SiO2 glass–ceramics was investigated. The result revealed that a small addition of CeO2 promotes the phase transformation from quartz to cristobalite. Moreover, the CeO2 dopant reduces the sintering activation energy and thus improves the densification of CAS system. Also, the activation energy of crystallization is decreased and the crystallinity is elevated due to the modification of CeO2. Thus, in the case of 1 wt% CeO2 doping, the flexural strength and Young’s modulus of CAS glass–ceramics are, respectively, enhanced to 205 MPa and 78 GPa, accompanied by the increase in coefficient of thermal expansion from 10.0 × 10–6/°C to 12.4 × 10–6/°C.

Hot pressing sintering process and sintering mechanism of W–La2O3–Y2O3–ZrO2

The W–La2O3–Y2O3–ZrO2 materials were prepared by vacuum hot pressing sintering process. The microstructure was characterized by scanning electron microscopy (SEM). The effects of sintering temperature, sintering pressure, holding time and heating rate on relative density, hardness and microstructure were studied. The sintering activation energy of the powder was calculated. Based on the result, the best sintering parameters by vacuum hot pressing sintering are as follows: sintering temperature of 1600 °C, sintering pressure of 60 MPa and holding time of 60 min. The heating rate is 10 °C·min−1 from room temperature to 1000 °C and 4 °C·min−1 from 1000 to 1600 °C. Using this process, the grain size is about 3 μm in diameter with relative density at 99.7%, and the hardness is HV 528.5. The sintering characteristic index (n) of the specimen is 4, and the sintering mechanism mainly depends on the diffusion from the surface to the intragranular in the volume diffusion. The sintering activation energy of W–La2O3–Y2O3–ZrO2 material is 286.2 kJ·mol−1, which indicates that the powder has higher activity in sintering process.

Microstructure and mechanical properties of Mo–Cu–Zr composites fabricated via microwave sintering

Mo–Cu composites exhibit excellent physical and mechanical properties, while the problems of densification and interface seriously limit their application. In this paper, the effect of element Zr on the properties of Mo–Cu composites is investigated aiming to overcome these critical issues. Various compositions of Mo–Cu–Zr ternary composites were fabricated via ball milling, cold isostatic pressing and microwave sintering. Microstructures and mechanical properties of the prepared composites were systematically characterized and analyzed. The results show that the density and mechanical properties of the composites increase with the increase of Zr content, and the 50Mo30Cu20Zr composite achieves the highest relative density of 97.6%, hardness of 314.4 HV and compressive strength of 805.9 MPa. The increase of Zr content reduces the sintering activation energy and leads to the improvement of density. Simultaneously, the element Zr can form solid solutions and in-situ synthesis intermetallic compounds with Cu matrix, which leads to solid solubility strengthening and dispersion strengthening. Moreover, the interface between Mo and Cu phase is found to have been reinforced by the in-situ intermetallic compound particles embedded in the interface. This work provides an important reference for research of densification and mechanical property of Mo–Cu composites.

Densification kinetics and sintering behaviour of a Ti(C0.7N0.3)-WC-Mo2C–NbC-(Co,Ni) compact

In order to understand the densification kinetics of a Ti(C0.7N0.3)-WC-Mo2C–NbC-(Co, Ni) compact, the linear shrinkages were measured using a horizontal dilatometer at different heating rates (2, 4, and 6 °C/min) in an argon atmosphere. Densification is more efficient when it is sintered at a lower heating rate. A master sintering curve was constructed over a range from the relative density of approximately 0.658–0.816; the corresponding sintering activation energy was 630 kJ/mol. With the aid of an Arrhenius curve, the sintering behaviour of cermet was analysed at different temperatures. The sintering process changed from solid-phase to liquid-phase sintering at approximately 1385 °C. The sintering activation energy turned negative when the temperature exceeded 1385 °C at a heating rate of 2 °C/min due to the formation of a liquid phase, and the exothermic process of dissolution-precipitation.

The Master Sinter Curve and Its Application to Binder Jetting Additive Manufacturing

AbstractThe master sinter curve (MSC) is an empirical model used to predict the density of a part after being sintered. The model is typically applied to components that undergo isotropic shrinkage. Parts manufactured using binder jetting additive manufacturing (BJAM) are known to have nonuniform powder systems and high levels of anisotropy. This work explores the application of the master sinter curve to components made by BJAM. Cylindrical samples were manufactured with the long axis parallel (vertical), perpendicular (horizontal), and 45 deg to the printing direction. A bimodal blend of titanium powder (0–45 µm and 106–150 µm) was used to make samples with consistent green densities (ranging from 47.2% to 52.3%) between the different orientations. Samples were then sintered at heating rates of 1, 3, and 5 °C/min to a maximum of 1400 °C. After sintering, the samples showed significant variation between the different orientations, with vertical samples on average 7.6 ± 2.98% and 4.7 ± 1.20% denser than the horizontal and the 45 deg samples, respectively. The calculated apparent activation energies for sintering were within the same range for all orientations, 200–260 kJ/mol for vertical and 45 deg, and 140–260 kJ/mol for horizontal samples. Validation sinter runs showed that the density prediction errors of the master sinter curves were between 0.9% and 4.3%. This work shows that the master sinter curve can be applied to predict the sintered density of components manufactured by binder jetting additive manufacturing.

Investigation on low-temperature reactive viscous flow sintering behavior of lanthanum-borate glass-ceramic with BaTi4O9 ceramic filler

In this paper, the sintering behavior of the lanthanum-borate (B2O3-La2O3-MgO-TiO2,BLMT) glass-ceramic with BaTi4O9 filler composite was investigated in terms of the wetting behavior, interfacial reaction, sinter-shrinkage process, sintering activation energy, as well as phase and microstructure evolution with change in filler contents and sintering temperature. Our research suggested that the glass is unable to wet the filler material within a temperature up to 1000 °C, indicating that the densification process of composites is dominated by viscous flow of glass matrix. The increase in filler content that performs as a rigid particle in composite causes the rise in the sinter-shrinkage-onset and -end temperature, thereby proving that the viscous-flow densification of the composites with x≤ 30 wt% filler content is accomplished before the crystallization of BLMT glass, whereas the composites with x≥ 40 wt% cannot. After densification, a chemical reaction that almost synchronizes with the glass crystallization occurred between glass and ceramic, which not only imposes significant influence on the crystallization behavior, but eradicates the closed pore formed by the viscous flow and the induced crystallization porosity. The densification process of the BaTi4O9 filler-BLMT glass composite was referred to as two-stage reactive assisted viscous flow sintering process which consists of viscous flow of glass, of the crystallization process of glass, and/or of the chemical reaction between glass and filler.

The influence of heating rate on W-Cr-Zr alloy densification process and microstructure evolution during spark plasma sintering

In this work, W-Cr-Zr as a self-passivating tungsten alloy is studied. Spark plasma sintering (SPS) is used to prepare the sample. The influence of the heating rate on the densification process and microstructure evolution during SPS was investigated. The increasing the heating rate enhance the homogeneity of the microstructure, however, did not show great effect on overall the sample density. High heating rate with high thermal inertia led the actual temperature to approach or even exceed WCr alloy solution temperature. Based on the punch displacement curves, it can be found that the densification temperatures are approximately 1380 °C and the high heating rate could promote the densification rate. On the other hand, high heating rate required high current, which can decrease the sintering activation energy and improve the densification process.

Kinetics of Spark Plasma Sintering of WC–10% Co Ultrafine-Grained Hard Alloy

The effect of the carbon content on the kinetics of spark plasma sintering of nanopowder and submicron powder of WC–10% Co compositions was studied. Free carbon in the graphite form was introduced into nanopowders and submicron powders by mixing. The activation energy of solid-phase sintering at isothermal holding and continuous heating was determined. It is shown that increase in the carbon content leads to decrease in the volume fraction of the η phase and shift of the shrinkage curve into the region of low temperatures. It is established that an increase in the carbon content does not have a significant effect on the sintering activation energy of nanopowders and submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It is shown that an increase in the carbon content leads to a significant decrease in the activation energy of consolidation of WC–Co powders in the region of “high” sintering temperatures owing to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The sintering kinetics of fine-grained WC–Co alloys is limited by the rate of Coble diffusion creep of cobalt.

Fabrication of high‐density NiFe2O4 ceramics by slip casting and pressureless sintering

ABSTRACTHigh‐density NiFe2O4 ceramics with homogeneous microstructure were produced by slip casting and pressureless sintering. The slurry stability, sintering behavior, and microstructure of NiFe2O4 ceramics were investigated. A stable slurry can be obtained by adding 12.5 wt% NiFe2O4 nanoparticle and 5 wt% nano‐binder at a slurry pH around 11.0. The linear shrinkage and linear shrinkage rate for both NiFe2O4 ceramic green bodies shaped by cold press molding and slip casting showed nearly the same trends. The temperature associated with the maximum linear shrinkage rate of slip casted green body was 1263.5°C, which was lower than that of cold press molded sample (1272.0°C). The sintering activation energy of slip casted green body was also lower than that of cold press molded sample (279.18 vs 288.47 kJ mol−1), owing to high density and homogeneity of slip‐casted green compact. A high‐density NiFe2O4 ceramics with uniform grain size distribution can be produced by slip casting and pressureless sintering at 1350°C for 6 hours, attributed to the ability of slip casting to minimize agglomerates and micropores. It demonstrated that slip casting was more suitable to prepare high‐density NiFe2O4 ceramics with good homogeneity.

Low‐temperature sintering of CaMgSi2O6-KBS composites with ultralow dielectric constant

In this paper, ultralow dielectric constant CaMgSi2O6-KBS composites (εr < 5) were fabricated via the solid-state route. The sintering temperature (ST) of CaMgSi2O6 ceramics was successfully reduced from more than 1200 °C–850 °C with assistance of the K2O–B2O3–SiO2 (KBS) glass. The impacts of KBS glass on phase compositions, microstructure, sintering activation energy and microwave dielectric properties of CaMgSi2O6 ceramics were systematically discussed. The X-ray diffraction results confirmed that the (1-x)CaMgSi2O6-xKBS composites possessed CaMgSi2O6 phase and SiO2 phase. The results of Rietveld refinement showed that the weight fraction of CaMgSi2O6 phase gradually decreased with increasing KBS glass and sintering temperature. The (1-x)CaMgSi2O6-xKBS (x = 0.5) composite sintered at 850 °C exhibits optimal microwave dielectric properties: εr = 4.58, Q × f = 13989 GHz (13.95 GHz), τf = -22.9 ppm/oC. It is a promising material for low-temperature co-fired ceramic (LTCC) applications which desire for ultralow dielectric constant materials.

Microstructural evolution and sintering properties of palygorskite nanofibers

AbstractIn this study, the morphological evolution and sintering properties of the palygorskite nanofibers were studied along with the increase of temperature, using raw palygorskite as materials. The palygorskite powder was calcined at different temperatures in the range of 100°C‐1200°C, and the microstructural evolution of the palygorskite nanofibers was investigated by thermogravimetric and differential thermal analysis (TG‐DTA), X‐ray diffraction (XRD), scanning electron microscopy (SEM), and high‐resolution transmission electron microscope (HRTEM). Furthermore, the palygorskite powder was shaped to bars by dry pressing and sintered from 700°C to 1200°C. The properties of the sintered palygorskite were characterized by bending strength, mercury intrusion porosimeter (MIP), and stepwise isothermal dilatometry (SID). The results showed that the morphology of palygorskite nanofibers maintained unchanged till 1000°C. The palygorskite nanofibers molted to bind each other and formed a solid interwoven network structure at 1100°C. Correspondingly, it was shown from the sharply decrease of the sintered palygorskite porosity from 45.46% at 1000°C to 1.82% at 1100°C that the dense sintering of palygorskite started at 1100°C. With the sintering proceeding, some closed micropores fused each other to form bigger opening pores, resulting in a slight increase of porosity at 1200°C. However, the pore size distribution got more uniform and the density of the sintered body increased. So the bending strength of the sintered body reached the maximum of 176.67 Mpa and finally the main crystalline phases of the sintered sample changed to quartz, enstatite, and kyanite. The sintering activation energy of the palygorskite was measured by means of SID with a value of 906.46 kJ·mol−1.

Study of the kinetics of spark plasma sintering of ultrafine-grained hard alloys WC-10%Co

The effect of carbon on the kinetics of spark plasma sintering (SPS) of submicron powder compositions WC-10wt.%Co is investigated. Free carbon in the form of graphite was added into submicron powders by mixing. The activation energy of solid-phase sintering under the conditions of isothermal hold and continuous heating rate sintering was determined. It was shown that an increase in the carbon content leads to a decrease in the volume fraction of particles of the η-phase and a shift of the shrinkage curve to the region of lower heating temperatures. It was established that an increase in the graphite content does not significantly affect the activation energy of sintering of submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It has been shown that an increase in graphite content leads to a significant decrease in the activation energy of compaction of WC-Co powders in the region of “high” sintering temperatures due to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The kinetics of sintering of fine-grained WC-Co hard alloys is limited by the intensity of the cobalt diffusion creep by Coble.

Электроимпульсное плазменное спекание нанопорошков WC – 10 Co с различным содержанием углерода, полученных методом плазмохимического синтеза

The features of high-speed sintering of WC – Co nanopowders with various contents of excess carbon (colloidal graphite) were studied. To obtain powders, a process was used that included plasma-chemical and low-temperature syntheses and a chemical-metallurgical method of applying ultrathin cobalt layers by precipitation from a solution of salts. The consolidation of powder materials was carried out by the method of high-speed Spark Plasma Sintering. It was found that an increase in the concentration of free carbon (colloidal graphite) has the greatest effect on the shrinkage and sintering rate at the stage of intense shrinkage of WC-Co nanopowders. It is shown that an increase in the carbon content in the composition of nanopowders leads to a decrease in the value of sintering activation energy at the stage of intense shrinkage.It has been established that the process of nanopowder compaction at the intense shrinkage stage is determined by the intensity of the plastic flow and the grain boundary diffusion of cobalt. It is shown that the mechanism of plastic deformation of the γ-phase based on cobalt corresponds to the Coble diffusion creep. It was found that an increase in carbon content leads to decreased in activation energy at the intense shrinkage and does not significantly affect at stage III of sintering where decrease in the shrinkage intensity is observed. It was shown that a decrease in the sintering activation energy is due to a decrease in the tungsten concentration in the γ-phase.

Study on Sintering Kinetics of (Th x Ce1-x )O2 Ceramic Pellets Prepared via Sol-Gel Method

(ThxCe1-x)O2 microspheres (x = 0.50, 0.75, and 0.95) prepared by sol-gel microsphere technique were compacted to pellets. The sintering kinetics, diffusion mechanism, and activation energy of the (ThxCe1-x)O2 pellets were investigated by dilatometry for 1100°C, 1200°C, and 1300°C. The rate controlling sintering method, one of the most sensitive methods, was chosen to investigate the sintering kinetics. The pellets were heated with a rate of 10°C/min and were held for 10 h at the above mentioned temperatures under isothermal conditions. The activation energies for the (Th0.50Ce0.50)O2, (Th0.75Ce0.25)O2, and (Th0.95Ce0.05)O2 pellets were calculated as 305, 315, and 419 kJ·mol−1, respectively. In the experiments, green densities of the mixed-oxide pellets were determined as 45% to 47% of the theoretical density for all of the studied ratios. Sintering densities reached up to 94% of theoretical density after sintering at 1300°C. Scanning electron microscopy images of the pellets were taken.

Manufacturing of fine-grained titanium by cryogenic milling and spark plasma sintering

Commercially pure (CP) gas-atomized titanium powder was processed by wet cryogenic milling in liquid argon and compacted by spark plasma sintering. The time-dependent sintering evolution at different temperatures was evaluated by using the master sintering curve (MSC) approach with the aim of achieving a material with maximum relative density and minimum grain size. Carrier-gas hot extraction (CGHE) confirmed a purity consistent with ASTM standard of Grade 4 CP Ti. Grain size and texture were determined by EBSD. An apparent activation energy of sintering of 115 kJ/mol was found based on the MSC approach. It is significantly lower than the activation energy of self-diffusion in Ti. This is attributed to an enhanced diffusion rate due to high concentration of defects in the powder after milling. The relative density was correlated with the resulting grain size and a general trade-off relationship between achieving high relative density and maintaining small grain size was found. The distribution of oxygen after milling and subsequent sintering at low temperatures is heterogeneous as determined by complementary XRD, CGHE and microhardness measurements. The distribution of oxygen becomes homogeneous with increasing sintering temperature. The microhardness of the material was shown to depend on residual porosity, the content and distribution of oxygen and also on texture. Processing parameters of milling and sintering were optimized to achieve fully dense, fine grained material with a low contamination by nitrogen and oxygen.

A differentiating approach to the master sintering curve analysis

The master sintering curve (MSC) has been widely applied in analyzing and predicting the densification process during sintering. The usual approach to the MSC analysis supposes that the apparent activation energy of sintering Q keeps constant during the whole sintering process, but some studies reported that the density influenced Q. This paper proposed a differentiating approach to the MSC analysis, in which Q was taken as a function of the density or the linear shrinkage of the compact. The MSC of a SiO2-CaO co-doped alumina ceramic during sintering was built by the differentiating approach and the usual approach. It was found that the differentiating approach was more accurate in predicting the densification process than the usual approach, and it was less time-consuming in the numerical computation than the usual approach. The evolution of Q with the density calculated by the differentiating approach can be used to analyze the effect of the density on the densification rate during sintering.

The sintering kinetics and properties of sintered glass-ceramics from coal fly ash of different particle size

Coal fly ash, a solid waste from powder plant, was fully used to produce sintered glass-ceramics by direct sintering in air after dry pressing at 10 MPa. The influence of particle size on the sinterability and properties of sintered glass-ceramics had been systematically discussed. Compared with CFA (d50 = 12.6 μm), sintered glass-ceramics issued from 20CFA (d50 = 7.5 μm) and 40CFA (d50 = 4.9 μm) had higher bulk density, higher linear shrinkage and lower porosity. The sintering activation energy of the sintered glass-ceramics obtained from the three sizes of coal fly ash (CFA, 20CFA and 40CFA) were estimated by bulk density and linear shrinkage, which were 127, 102, 81 kJ/mol and 124, 106, 83 kJ/mol respectively, meaning the smaller the particle size, the easier the sintering. According to the XRD analysis, anorthite was formed by the reaction of portion dissolved quartz with the amorphous metastable glassy phase in coal fly ash. The smaller the particle size of coal fly ash is, the easier the formation of anorthite. Moreover, the properties (water absorption, flexural strength and chemical resistance) of the sintered glass-ceramics were enhanced with the decrease of the particle size and reached the standards of architectural decoration materials.

Finite‐difference calculation on an uncertainty evaluation of the kinetic parameters from initial sintering of UO2

AbstractComputational simulation based on the one‐dimensional finite‐difference method was performed in order to clarify the effect of particle size and the coexistence of particles with different diameters on sintering kinetics of UO2. Particle arrays with different diameters were arranged in parallel, and sintering behavior, such as shrinkage and neck growth, was calculated by the finite‐difference method. The shrinkage of the particle array and the stress on the grain boundary were calculated in the linked sintering. The results indicated that evaluation of the activation energies for sintering of compacts of UO2 seems likely to be possible by selecting an appropriate shrinkage range. However, evaluation of the diffusion coefficients using compacts is questionable.

Impact of high energy ball milling on densification behaviour of magnesium aluminate spinel evaluated by master sintering curve and constant rate of heating approach

The present study investigates the influences of high energy ball-milling on densification of a commercial sub-micron magnesium aluminate spinel powder. High energy milling decreases the crystallite size and modifies the particle size distribution of powder resulting in better compaction of green bodies. Milling, also, increases the number of structural defects.The concept of master sintering curve (MSC) and the constant rate of heating (CRH) approach were employed to evaluate the activation energy of sintering. It was demonstrated that, according to MSC, the milled samples exhibit lower densification activation energy compared to the as-received powder (530 kJ mol−1 vs 750 kJ mol−1). Analysing the densification using CRH approach reveals that the sintering activation energy of milled samples remains constant during the sintering process, while that of as-received samples increases with densification. The estimated activation energy using MSC method is thus an average value of the values measured over the whole process of sintering.

Microwave sintering of Mo nanopowder and its densification behavior

Abstract In order to prepare high-performance Mo with fine and homogeneous microstructure to meet the demand of high-technology applications such as metallurgical, mechanical, national defense, aerospace and electronics applications, the microwave sintering process and densification mechanism of Mo nanopowder were studied. In this experiment, Mo nanopowder and micropowder were used for conventional sintering and microwave sintering at different sintering temperatures and sintering time, respectively. The results showed that with the increase in the sintering temperature, the increase rates of the relative density and hardness increased rapidly at first and then slowed down. The relative density rapidly reached 95%, followed by a small change. Mo nanopowder with a relative density of 98.03% and average grain size of 3.6 μm was prepared by microwave sintering at 1873 K for 30 min. According to the analysis of the sintering kinetics, its densification is attributed to the combination of volumetric diffusion and grain boundary diffusion mechanisms. The calculated sintering activation energy of Mo nanopowder was 203.65 kJ/mol, which was considerably lower than that in the conventional sintering, suggesting that the microwave sintering was beneficial to the enhancement in the atom diffusion and densification for the powder. The results confirm that the microwave sintering is a promising method to economically prepare molybdenum with high properties.